This paper examines underachievement, a widely used term in education policy and practice often used to refer to nations, home nations, and regions; types and sectors of schooling; physiological, ethnic, and social groups; and individuals. It has been used to mean simply low achievement, also lower achievement relative to another of these groups, and lower achievement than would be expected by an observer. This paper presents examples of each. These multiple uses lead to considerable confusion which, coupled with common errors in assessing the proportionate difference between groups, mean that significant public money has been spent attempting to overcome problems that may not exist. When underachievement is understood to mean a lower achievement level by an individual (or group) than would be expected using a model based on the best available predictors, there is nothing that can be known about underachieving individuals (or groups) that they have in common. They cannot be disproportionately working class males, for example, because class and sex would then be part of the best available predictors. Even if some predictors were reserved from the best model, there is no evidence that underachievers have much in common. In raw score terms, it can be said that a particular social group exhibits lower achievement than another. It can also be said that there is a differential attainment between groups. However, this is not saying that the lower attaining group could and should do better on that assessment, or that the surface dissimilarity is the cause of differences in attainment. Making explicit what is meant by underachievement is essential. (Contains 67 references.) (SM)